1. Field of the Invention
The present invention relates to an apparatus and method for controlling air/fuel mixture ratio for an internal combustion engine in a feedback mode.
2. Description of the Background Art
A Japanese Patent Application First Publication No. Showa 60-240840 exemplifies a previously proposed air/fuel mixture ratio feedback controlling apparatus.
In the above-identified Japanese Patent Application First Publication, an intake air flow quantity Q and engine revolution speed N are detected so as to calculate a basic fuel supply quantity Tp (=K.multidot.Q/N; K denotes a proportional constant). The feedback correction for the basic fuel supply quantity Tp according to the correction by means of an engine temperature is carried out in response to an oxygen sensor detecting an air/fuel mixture ratio of the air/fuel mixture according to a detection result of oxygen concentration in the exhaust gas. Then, a final fuel supply quantity T.sub.I is determined according to the Tp which is corrected with a variation in a battery voltage.
Specifically, since the output signal of the oxygen sensor is inverted as a stoichiometric air/fuel mixture ratio as a boundary, the inversion of the output signal therefrom causes an increase or decrease of the fuel supply quantity so that a feedback correction coefficient by which the basic fuel supply quantity Tp is to be multiplied is set by means of a proportional-integral control (PI control). Thereafter, a drive pulse signal whose pulsewidth corresponds to the finally set fuel supply quantity T.sub.I is output from a control unit at a predetermined timing of fuel supply so that a previously determined quantity of fuel is injected into the engine.
The air/fuel mixture ration feedback correction based on the air/fuel mixture ratio detector (oxygen sensor) is carried out such that the air/fuel mixture ratio is controlled to approach to the target (stoichiometric) air/fuel mixture ratio. This is because in order to secure a high exhaust gas purification performance, a conversion efficiency (purification efficiency) of a three-catalytic converter installed in the exhaust gas passage so as to oxidize CO and HC (hydrocarbon) and so as to reduce N0.sub.x can effectively function in an exhaust gas state during a combustion of the stoichiometric air/fuel mixture.
Then, proportional component and integral component are set respectively, for example, according to a deviation between the air/fuel mixture ration detected by the oxygen sensor and the target air/fuel mixture ratio and the added value of the proportional and integral components is multiplied by the basic fuel supply quantity Tp as a value of .alpha. so that the air/fuel mixture ratio is controlled to a position placed in a proximity to the stoichiometric air/fuel mixture ratio.
In the air/fuel mixture feedback control system or apparatus described above, the feedback control of the air/fuel mixture ratio based on the output signal of the oxygen sensor is started after a predetermined time from a confirmation of state in which an activation of the oxygen sensor is determined and a preferable output characteristic is obtained upon the lapse of the predetermined time.
However, an industrial demand has been issued to promote the exhaust purification performance during an interval from a cold state to a warmed up state with the air/fuel mixture feedback control carried out as early as possible.
To make the start of the air/fuel mixture ratio earlier than usual, it is necessary to start the air/fuel mixture ratio immediately after an activation determination of the oxygen sensor such that an activation of the oxygen sensor is determined in the vicinity to a limit of the output signal of the oxygen sensor. However, when the activation of the oxygen sensor is insufficient, a point at which the output signal of the oxygen sensor is inverted is shifted away from the stoichiometric air/fuel mixture ratio so that it is impossible to carry out the feedback control in the vicinity to the stoichiometric air/fuel mixture ratio. A direction of the shifted point is generally toward a lean side although the shifted point is different according to the characteristics of the oxygen sensor.